用于不稳定原子核研究的共线激光谱仪研制进展

Progress on the development of the collinear laser spectroscopy setup for the study of unstable nuclei

不稳定原子核的基本性质是研究核结构的重要探针之一,有助于我们理解丰中子/丰质子核中出现的新奇物理现象,也是检验和发展核理论模型的重要依据.利用精密激光谱学技术测量核外电子的超精细结构光谱,可以精确提取不稳定原子核基态和长寿命同核异能态的自旋、磁矩、电四极矩和电荷半径等多个基本性质.本文简单介绍了国际上激光谱学技术的发展现状和超精细结构光谱的测量原理.在此基础上,详细介绍了在我国研制的基于荧光探测的共线激光谱系统及近期的优化升级现状.通过测量Ti原子和Ca离子的高分辨共振谱,有效验证了共线激光谱设备的整体性能.最后,结合我国现有和发展中放射性核束装置的物理目标,提出进一步优化升级激光谱技术,提高其分辨率和灵敏度,并用于不稳定核研究的计划.

The basic properties of unstable nuclei are very sensitive probes for the study of exotic nuclear structures,which is of importance for an in-depth understanding of the unexpected structure phenomena in atomic nuclei with large proton-toneutron ratios.Collinear laser spectroscopy(CLS)techniques have been proven to be a powerful tool to determine simultaneously multiple fundamental properties of ground and long-lived isomeric states of exotic nuclei,such as nuclear spins,electromagnetic moments and charge radii,by measuring the subtle hyperfine structure and isotope shifts in a nuclear model-independent manner.The studies of nuclear properties and structures of unstable nuclei using laser spectroscopy have been broadly carried out at ISOL(Isotope Separator On-Line)-type radioactive ion beam(RIB)facilitates,such as ISOLDE at CERN,IGISOL at JYFL and ISAC at TRIUMF,where the low energy RI beam is directly offered with highquality.With the goal to explore more exotic nuclei,laser spectroscopy techniques,with their continuous technological development towards higher resolution and higher sensitivity,have been broadly established and planned at current-and next-generation RIB facilities worldwide.This CLS technique,however,has not yet been implemented at current operational domestic RIB facilities,the PF(Projectile Fragmentation)-type HIRFL at IMP and the BRIF(ISOL-type)at CIAE,for the studies of nuclear properties of unstable nuclei.Thus,we have recently developed a fluorescence-detectionbased CLS system,which has been commissioned with stable ion beam produced from an offline laser ablation ion source and radioactive ion beam from BRIF RIB facility.In this paper,we briefly introduce the recent progress on the development of laser spectroscopy techniques at RIB facilities worldwide,as well as the experimental methods to measure the hyperfine structure spectra.Subsequently,the details of the CLS system recently developed,are presented together with recent upgrades and offline test.The optimization and upgrade of this system,include the lens rotator system for laser spot control,ion beam imaging using a phosphor screen,and calibration of the neutralization efficiency of the change exchange process.Based on these upgrades,we performed the offline test experiment using stable Ca and Ti beams.The optical spectra for the 4s ^(2)S_(1/2)→4p ^(2)P_(3/2) ionic transition of ^(40)Ca and for the 3d^(2)4s^(23)F_(2)→3d^(3)4p^(3)D_(1) atomic transition of48Ti were obtained,giving a spectral linewidth of approximately 55 MHz for Ca and 65 MHz for Ti.The total detection sensitivity estimated in the case of 40Ca is comparable to that of the well-established CLS setups worldwide.Finally,a summary and outlook on the further development of the collinear resonance ionization spectroscopy towards higher sensitivity are given,which will be used at the current and future RIB facilities in China for the study of unstable nuclei with shorter half-lives and lower production yield.Further implementation of the UC,target into the BRIF facility is currently on-going,which will potentially produce more exotic isotopes at different mass regions.Development and installation of a radiofrequency quadrupole cooler and buncher are also on-going,which could provide bunched ion beams with low energy spread of few electron volts,being essential for the high resolution and high sensitivity measurement of unstable nuclei using CLS.